Pybus Formation and correlative? limestone

Unit symbol: Plps
Age range Permian (298.9 to 259.9 Ma)
Lithology: Sedimentary
Group name: Limestone and calcareous clastic rocks
Characteristically “light-brownish-gray to white, medium-bedded, fossiliferous, fine- to medium-grained dolomite [dolostone], which contains irregular layers and scattered angular fragments of bluish-white chert. The chert content increases upward and near the top of the formation constitutes as much as 90 percent of the rock” (Lathram and others, 1965). On northern Kuiu and Kupreanof Islands and Admiralty Island, unit consists of medium-bedded dolostone, limestone, and subordinate gray chert beds and nodules (Lathram and others, 1965; Muffler, 1967; and Brew and others, 1984). The Pybus Formation contains very late early Permian conodonts (Premo and others, 2010), Cisuralian (early Permian) brachiopods (Muffler, 1967; Brew and others, 1984), and Guadalupian (middle Permian) conodonts (Karl and others, 1999). In the Glacier Bay area, light- to medium-brownish-gray, fine- to medium-grained crystalline, fossiliferous limestone contains sparse, paper-thin, reddish brown separations and is overlain by nodular bedded, light-brownish-gray, fine- to medium-grained limestone interbedded with light-greenish-brown chert. The upper limestone and chert varies along strike from an interbedded sequence to apparent limestone-chert conglomerate, which is, in turn, overlain by dark-gray slate, interbedded with very fine-grained and silty calcareous graywacke (Brew and Ford, 1985)

Source map information

Source map Karl, S.M., 2013, Unpublished data.
Symbol Pp
Unit name Pybus Limestone
Description On Admiralty Island, Pybus consists of light gray to tan, medium- to thick-bedded fossiliferous dolostone and limestone, where it was first named by Loney (1964). Dolostone contains lenses and nodules of blue to gray chert. There are blue-gray chert beds to 20 cm thick at the top of the unit. Chert contains silicified megafossils, as well as radiolarians and sponge spicules.Maximum unit thickness is about 300 m. In Keku Straits area, Pybus Formation consists of limestone, dolomite, and chert; conspicuous cliff-forming medium-bedded to massive non-bedded coarsely crystalline white to very light gray dolomitic limestone with light gray replacement chert as thin beds, nodules, fragments, and crosscutting masses; minor coarse-grained light gray limestone, fetid medium-gray dolomite near top of unit. In Keku Straits also includes Halleck Formation siltstone, sandstone, limestone, and conglomerate. At Saginaw Bay, unit consists dominantly dark-gray very calcareous siltstone that grades into silty limestone, minor intercalated thin beds of fossil fragmental limestone, and very fossiliferous medium-bedded crossbedded medium-gray weathering calcareous sandstone, some small bioherms. In Keku Islets, dominantly thick-bedded polymictic pebble and cobble conglomerate with rounded clasts of chert, felsic volcanic rock, and minor limestone derived mostly from Cannery Formation; minor dark-gray calcareous siltstone and thin- to medium-bedded medium gray fine- to coarse-grained crossbedded sandstone; complexly folded locally. On Kupreanof Island, includes light gray to buff, medium-bedded, fossiliferous limestone and dolostone. Contains chert as thin beds, nodules, and crosscutting diagenetic masses
Lithology Sedimentary

Correlated geologic units

Label Pp
Description Pybus Formation, limestone and dolostone
Geologic age Artinskian to Guadalupian
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Dolostone < Carbonate < Sedimentary Bed Major
Limestone < Carbonate < Sedimentary Bed Major
Chert < Chemical < Sedimentary Bed Minor